Ir-light emitters based on swnt&#39;s (single walled carbon nanotubes), semiconducting swnts-light emitting diodes and lasers

ABSTRACT

The present invention relates to a new light emitters that exploit the use of semiconducting single walled carbon nanotubes (SWNTs). Experimental evidences are given on how it is possible, within the standard silicon technology, to devise light emitting diodes (LEDs) emitting in the infrared IR where light emission results from a radiative recombination of electron and holes on semiconducting single walled carbon nanotubes (SWNTs-LED). We will also show how it is possible to implement these SWNTs-LED in order to build up a laser source based on the emission properties of SWNTs. A description of the manufacturing process of such devices is also given.

BACKGROUND

1. Technical Field

The present invention relates to a light-emitting device including ap-insulator or a Si-n nanojunction region.

2. Description of the Related Art

Carbon nanotubes (CNTs), discovered by Iijima in S. Iijima Nature 354 56(1991), as a by-product of the fullerene soot, are carbon hollowcylindrical-shaped molecules that, having a diameter of few nanometersand a length ranging in the micron scale, can be considered as molecularnanowires.

Their properties are determined by the number of graphene sheets (walls)forming a nanotube and by the fashion their atoms arrange in a wall. Inparticular, with respect to the number of graphene-sheets forming ananotube, carbon nanotube basically occur in two distinguished types:MWNTs (Multi Walled NanoTubes) and SWNTs (Single Walled NanoTubes).Whereas, SWNTs are further characterized by the arrangement of hexagons,described by the chiral vector C, forming the honeycomb structure of thewrapped graphene sheet, with respect to the axis of the tube.

SWNTs have emerged in the field of molecular electronics because oftheir unique properties that allow for the manufacturing of devices suchas FETs (field effect transistors) and SETs (single electrontransistors). See in this respect the prior works by: S. J. Tans, A. R.M. Verschueren and C. Dekker Nature 393 49 (1998); R. Martel, T.Schmidt, H. Shea, T. Hertel and Ph. Avouris Appl. Phys. Lett. 73 2447(1998); and S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R. E. Smalley,L. J. Geerligs and C. Dekker Nature 386 474 (1997).

As a matter of fact, with respect to the conduction properties, SWNTscan be either semiconducting or metallic nanotubes, depending on the‘wrapping’ of the graphene sheet (i.e., on their chiral vector). See inthis respect: H. W. Ch. Postma, T. Teepen, Z. Yao, M. Grifoni and C.Dekker Science 293 76 (2001); M. Bockrath, D. H. Cobden, P. L. McEuen,N. G. Chopra, A. Z. A. Thess and R. E. Smalley Science 275 1922 (1997);A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, H.Lee, S. G. Kim, D. T. Colbert, G. Scuseria, D. Tomanek, J. E. Fischerand R. E. Smalley Science 273 483 (1996).

On the other hand, the recent finding of light emission or fluorescencein the near IR region from semiconducting SWNTs and FETs based on SWNTsincreases and spreads the interest about the optical properties of thesematerials.

Because of the potential applications that would result from theexploitation of the optoelectronic properties of SWNTs (i.e., thepossibility of building new devices), it is worthwhile to study theinteraction of carbon nanotubes with radiation, especially in the nearIR region that is of major interest in the field of telecommunication.Article of some interest are the following: M. J. O'Connell, S. M.Bachilo, C. B. Huffman, V. C. Moore, 1 M. S. Strano, E. H. Haroz, K. L.Rialon, P. J. Boul, W. H. Noon, C. Kittrell, J. Ma, R. H. Hauge, R. B.Weisman, R. E. Smalley, Science 297 593 (2002); Sergei M. Bachilo,Michael S. Strano Carter Kittrell, Robert H. Hauge, Richard E. Smalley,R. Bruce Weisman, Science 298 2361 (2002); J. A. Misewich, R. Martel,Ph. Avouris, J. C. Tsang, S. Heinze, J. Tersoff, Science 300 783 (2003).

BRIEF SUMMARY

One embodiment of the present invention provides a light emitting diode(LED) emitting in the infrared IR spectrum where the light emissionresults from a radiative recombination of electron and holes onsemiconducting single walled carbon nanotubes (SWNTs-LED), by usingstandard silicon technology. In other words, one embodiment of thepresent invention is that of implementing a SWNTs-LED in order to buildup a laser source based on the emission properties of SWNTs.

One embodiment of the present invention focuses on the optical responseof SWNTs, and in particular the optical behaviour of opportunelypurified SWNTs deposited on a Si substrate in the infrared region (IR),by using several and also complementary techniques, such as the FT-IRspectroscopy (Fourier transformed infrared spectroscopy), the Ramanspectroscopy and compare these results with the optical response ofSWNTs in the ultraviolet, visible and near infrared (UV-vis-near IR)region, which is of interest because of the energy gap of semiconductingSWNTs.

One embodiment of the present invention provides for a light-emittingdevice including a p-insulator or a Si-n nanojunction region andcomprising at least a layer including semiconducting single walledcarbon nanotubes (SWNTs).

Advantageously at least two p and n doped silicon electrodes delimit avery thin insulating dielectric film wherein semiconducting SWNTs areembedded.

Another embodiment of the present invention provides the manufacturingprocess of such a device.

The features and advantages of the light emitting diode and itsmanufacturing method will be apparent from the following description ofan embodiment thereof given by way of non-limiting example withreference to the attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 schematically shows an interface formed by a Si p⁻ substrate,with its native oxide or a Si thin layer, and an ensemble of metal andsemiconducting SWNTs deposited on the Si surface; and wherein holeinjection is possible according to the difference in the work functionsbetween Si and SWNTs;

FIG. 2 is schematic view of a P-insulator or (Si)-n nanojunction withthe SWNTs embedded;

FIG. 3 is schematic view of a band diagram of the LED based on SWNTsaccording to the present invention with biased electrodes;

FIG. 4 is schematic view of a band diagram under equilibrium conditions;

FIG. 5 is a schematic view of a Fabry-Perot microcavity, comprising forexample two distributed Si/SiO₂ Bragg reflectors, wherein the Si/SiO₂layers and the central region can be obtained by annealing the filmsdeposited by PECVD;

FIG. 6 is a photograph of a SWNTs deposited on Si p⁻ wafer, along the(100) orientation, wherein the aspect of a grain of the pristine sootbefore the purification process is shown in the insert (b);

FIG. 7A is a high resolution TEM micrograph showing the structure of thebundles of SWNTs;

FIG. 7B is a transmission spectrum in the IR region of SWNTs depositedon Si (100) p;

FIG. 7C is a Raman spectrum in the region of 1500-1650 cm⁻¹ of a SWNTson Si (100) p⁻;

FIG. 8A shows a diagram of the transmittance as a function of thewavelength in the UV-vis and near IR regions of SWNTs deposited on a Sisubstrate; and

FIG. 8B shows the tight binding calculation devised for zig-zag (n,0)nanotubes, using γ=2.75.

DETAILED DESCRIPTION

With reference to the drawings, and specifically to the example of FIG.3, a light emitting device according to the present invention isglobally and schematically shown with the numeral reference 1.

This device 1 may be a light emitting diode employing SWNTs as interfacebetween two doped semiconductor regions.

Until now it lacks a genuinely electroluminescent device based on carbonnanotubes. According to the present invention, it is possible to buildup such a device as a LED and a laser source where SWNTs are used as anactive material.

According to one embodiment of the invention a light emitting diode(LED) is provided with a laser based on single walled carbon nanotubes(SWNTs) and semiconductor technologies, emitting radiation in the nearinfrared (IR) range (760 nm-2500 nm).

Experimental evidences of possible charge injection from a doped siliconsubstrate to a thin film of purified SWNTs suggests the employment ofSWNTs as interface between two suitably doped semiconductors in whichthe injected holes and electrons can be recombined. This involvesseveral applications and hereinafter at least two related embodimentswill be disclosed based on this idea.

Up to now technology lacks of a light emitting device that can be easilyand cheaply grafted on silicon substrate. Moreover, it has only beenpossible to build up light sources for fiber optics from indiumphosphide (InP), however, miniaturized circuits based on InP chip havenot yet been manufactured. Thus, device 1 may find several applications,such as: fiber-optic telecommunications, information processing (such asin compact-disc players) and optically interconnected circuits.Specifically, the inventive device is able to emit light with a1.5-micrometers wavelength required in optical telecommunications, andhave the advantage of being of much smaller size than a conventional InPchip.

Furthermore, GaAs lasers are typically used in information processing.However, they are not suitable for telecommunication applicationsbecause GaAs lasers emit with a wavelength of 0.85 micrometers, which isuseful only for short-distance communications. Unfortunately LEDs aretypically built on wafers of GaAs for the reason that it is hard tocreate good-quality GaAs films on silicon chips.

We will now show that device 1 can emit lights at the 1.5-micrometerswavelength, since we have theoretically calculated and measured a bandgap corresponding to this wavelength in the sample (interfaceSi/SiO₂/SWNTs).

FIG. 1 schematically shows an interface formed by a Si p⁻ substrate,with its native oxide or a Si thin layer, and an ensemble of metal andsemiconducting SWNTs deposited on the Si surface. We have investigatedthe behavior of this Si/SiO₂/SWNTs interface with respect to lightabsorption and observed that light absorption occurs at the gap ofsemiconducting SWNTs, for those nanotubes that have a gap within that ofsilicon.

In order to evidence the presence of a gap in a nanotube, holes must beinjected. This is accomplished by Si p⁻ injecting the holes because ofthe difference in the work functions between Si and SWNT. “Workfunction” as used herein refers to the energy required to release anelectron from a crystal lattice, such as that in Si or SWNT. On theother hand, semiconducting SWNTs that have energy gaps ranging in theinterval of 0.5-1.0 eV, have a direct band gap that determines theluminescence phenomena that have been observed so far. For example GaAsand InP are direct band gap semiconductors.

For this reason also in SWNTs, many have reported the presence ofluminescence and the possibility to build IR electroluminescent devicesthat emit light as a ‘by-product’ of their working as a field effecttransistor (FET). Therefore, according to one embodiment of theinvention, a semiconducting p-n junction is provided as the activematerial for LEDs and lasers. Holes and electrons are injected in thisjunction where they undergo a radiative recombination. This processoccurs because the junction is formed by direct band gap semiconductors.

Although it is possible to build up diodes based on Si p-n junction,these devices cannot work as LEDs, because of the indirect band gap ofsilicon. In fact, although the charge injection of holes and electronsoccurs, light emission is a low probability process. Hole-electronrecombination in silicon occurs in a non-radiative way.

On the other hand, semiconducting SWNTs having a direct band gap couldwork as an LED emitting in the IR region, if used as intermediatematerial between a p and an n semiconductor. Moreover, it is possible tobuild up a LED based on SWNTs, by exploiting the same technology thathas been developed for the silicon junctions.

The working principle of the invention is shown in FIG. 2. Device 1comprises a p-insulator or Si-n nanojunction where two siliconelectrodes, doped with p and n respectively, delimit a very thin (5-10nm) insulating dielectric film (i.e., silicon oxide, Si) including orembedding the SWNTs.

In this thin central dielectric film, the SWNTs, more specifically,semiconducting SWNTs are embedded.

Since the SWNTs have semiconducting features, such as those in directband gap semiconductors, it is very likely that a pair formed by anelectron and a hole that happens to encounter a nanotube will undergoradiative recombination at the gap. Charge injection is determined byapplying a voltage to the electrodes. Electrons and holes have to tunnelthe very thin insulator in order to achieve the radiative recombination.

One embodiment of the present invention further provides a Fabry Perrotmicrocavity by devising two Bragg-Reflectors obtained by alternating Siand SiO₂ layers each being a few 100 nm thick. A Fabry-Perot microcavityis shown schematically in FIG. 5 and comprises two distributed Si/SiO₂Bragg reflectors, wherein the Si/SiO₂ layers and the central region canbe obtained by the annealing of films deposited by PECVD. The thicknessof the Si (57 nm) and SiO₂ (141 nm) layers match λ₀/4 n, λ₀/2 n,respectively, where λ₀ is the intended value of the resonance (i.e.,equal to λ₀=832 nm) and n the refractive index. The central region playsthe role of an active medium, which in our case is formed by the lightemitting diode based on SWNTs where the Si electrodes are formed byhighly doped p and n silicon.

In this way a method to confine the emerging light in the cavity isobtained.

In order to achieve a laser effect, it's suitable to obtain an inversionof population that can be realized by using highly doped semiconductors.In this case our laser source will resemble that of an Esaki-Tunneldiode that will work as a light emitter, where an inversion ofpopulation is possible.

An Esaki Tunnel Diode is typically obtained from a p-n junction by usinghighly doped p and n semiconductors. In this case the depletion layer isabout 10 nm and electrons and holes can tunnel the barrier, giving riseto a tunneling current which depends on the density of the state. In ourcase, however, the tunneling current of electrons and holes willencounter the states determined by SWNTs, which differs from that of anEsaki Diode. As a result, we will have the radiative recombination ofholes and electrons, wherein, the characteristic Current (I) vs. Voltage(V) curve of the inventive diode will resemble that of the Esaki diodebut the energy will be converted in radiation i.e., light.

A further embodiment of the present invention provides a manufacturingmethod of the device 1.

After building a lower electrode, a small amount of SWNTs are depositedon the surface which is covered with the native oxide or by a very thinlayer of silicon oxide or Si (Si is an insulator for SWNTs).

SWNTs can be deposited after a purification process, or after a CVDgrowth, avoiding the oxidation of the Si surface.

After that, a thin layer of an insulator is grown to form a layer of afew nm thick (10 nm at maximum). On the top of this layer where theSWNTs are embedded, a complementary layer of Si is grown.

In order to build a laser, by using the standard technology, whichalternate the growth of Si and native oxide or any alternating structurethat has the property to confine light because of its reflectance, it ispossible to build a Fabry Perrot cavity tuned to the wavelength ofchoice that determines the resonance and hence the light that will belasing.

On the other hand in order to reach a high level of injection the twoelectrodes may be realized by using highly doped Si n and p.

The present invention is further illustrated by the followingnon-limiting examples. Unless otherwise noted, all scientific andtechnical terms have the meanings as understood by one of ordinary skillin the art.

EXAMPLE Example 1 Preparation of SWNTs

As shown in FIG. 6, the material that appears as gray-blackish soot inthe photographs requires a purification treatment in order to separatethe SWNTs from the amorphous carbon and the residues of the metalnanoparticles such as Ni and Y that are used as catalysts during thegrowth.

In order to remove these residues we adopted a purification methodconsisting of an oxidation process that uses hydrogen peroxide (H₂O₂)similar to that reported by X.-P. Tang, A. Kleinhammes, H. Shimoda, L.Fleming, K. Y. Bennoune, S. Sinha, C. Bower, O. Zhou, Y. Wu Science 288492 (2000).

In detail, a small amount of the pristine soot (11.2 mg) was mixed with100 ml of H₂O₂ (30%) and the blend was heated for 5 h under refluxconditions at the temperature of 100° C. The remaining solid residue wasfiltered (paper average pores dimension 2.7 μm) and rinsed in methanol(95%). After that the solid residue was mixed with 30 ml of deionizedwater and mildly sonicated for 20 min at 75° C. in order to obtain asuspension.

Samples were prepared by depositing few drops of the suspension on a Sip⁻ substrate, along the (100) orientation. Subsequently the samples weredried up overnight and analyzed according to standard techniques.

Routine structural characterization has been carried out by means of anFE-SEM Leo 1550 (field emission scanning electron microscope) having alateral resolution of about 4 nm. Optical characterization has beenachieved both in the infrared IR region as well as in the UV-Vis near IRregion. Spectra have been recorded, in the infrared (IR) region, byusing a spectrum one FT-IR (Perkin-Elmer), whereas Raman spectroscopyhas been obtained by means of a Raman Jobin Yvon U1000. Spectra in theUV-Vis near IR region have been obtained with a Lambda 900 UV-vis/NIRspectrometer (Perkin Elmer).

All the spectra have been recorded at room temperature. Moreover, thestructural characterization have been completed by transmission electronmicroscopy TEM analysis of the CNTs and have been achieved with a Jeol2010 FX (200 kV) microscope. In this case, some drops of the sonicatedsolution have been dried up on a microgrid plate (spacing≈10 μm) for theTEM analysis.

Scanning electron microscopy (SEM) has been used as the customarytechnique in order to achieve structural characterization.

In this respect, SEM has revealed an excellent and invaluable tool inorder to detect the presence of CNTs and investigate about thearrangements of CNTs deposited on the Si substrate.

FIG. 6 shows a SEM micrograph of a sample of carbon nanotubes obtainedfrom our suspension after deposition on a Si p⁻ (100) substrate. It ispossible to observe a set of entangled wire-like structures attributableto the presence of carbon nanotubes.

In the insert of FIG. 6, indicated with (b), is reported a SEMmicrograph of the pristine and not purified soot deposited oncrystalline Si, in order to compare the effect of the purificationtreatment.

A measurement of the diameter of the wire-like structures suggests thatthe wires are formed by bundles of nanotubes, the dimension of thesebundles being on the order of 10 nm (nanotubes diameter=1.3 nm).

Details on the structure of carbon nanotubes have been determined byusing the transmission electron microscopy (TEM). Samples, preparedstarting from the sonicated solution, reveal as before the presence ofwire-like structures. TEM analysis allows resolving the structures ofthe wires. In particular, a magnification of the wires (FIG. 6) shows asthese structures are formed by bundles of CNTs where rows of carbonatoms are arranged to form the walls of the nanotubes. Moreover thedistance separating the walls is about 1.3 nm which is the averagediameter of the SWNTs that form our material.

Example 2 Optical Properties of SWNTs

A thorough study of the optical properties determined by the presence ofSWNTs on a silicon substrate has been carried out in the infrared (IR)and near IR region, as well as in the visible and ultraviolet (UV)intervals. In order to achieve this purpose FTIR (Fourier transformedinfrared) spectroscopy results have been compared with its complementarytechnique, i.e., the Raman spectroscopy and after that completed withthe investigation achieved in the near IR and UV-Vis regions.

In particular, the optical response of the SWNTs deposited on a Sisubstrate to infrared light (range 400-4000 cm⁻¹) shows that thetransmittance as a function of the number of wavelength decreasesuniformly (see FIG. 7A) in agreement with the experimental data,evidencing the presence of characteristics in the region of 1300-1700cm⁻¹, attributed tentatively to the C═O, and C═C vibration modes,because of the oxidation process we have used.

Whereas, the signals in the region of 2800-3000 cm⁻¹ have been assignedto C-H modes. In spite of this behavior, the assignment of the peaks tothe IR-active modes of SWNTs (at 633, 1382, 1580 cm⁻¹) is notstraightforward, in fact only one distinct peak at 663 cm⁻¹ is likelydetermined by an IR-active phonon vibration mode of SWNTs.

The optical response in the IR region has been completed and comparedwith the Raman scattering determined by the SWNTs deposited on a Sisubstrate. Because of their all-carbon structure the E_(2g), A_(2g),A_(1g) vibration modes of SWNTs are Raman active and determine, in theIR region, a Raman scattering which gives complementary information withrespect to the IR Spectroscopy, allowing the identification of SWNTs.FIG. 7B reports the Raman shifts collected in the interval 1500-1650cm⁻¹ with respect to the green line (514.5 nm) of an Ar⁺-laser used asan excitation source.

This spectrum is characterized by the presence of three adjacent peaksat 1593, 1570 and 1555 cm⁻¹, going from the higher peak to the lowerones, respectively. The existence of these peaks is attributable to thetangential vibration modes determined by the graphite-like structurethat is also present in carbon nanotubes. Moreover, these peaks havebeen calculated as belonging to the E_(2g) and A_(1g) modes of the (10,10) and (17, 0) nanotubes.

Optical characterization of SWNTs deposited on Si, because of theabsorption of substrate in the UV-vis region, has been achieved in therange of wavelength of 200-2500 nm (UV-vis and near IR), by studying howthe signal reflected by silicon is affected by the presence of SWNTs onits surface.

In fact, because of the absorption at the Si-gap of the substrate andits thickness (500 μm), no light is transmitted in the UV-vis range.Nevertheless, the reflected signal (≧30% also in the near-IR region)suffices in order to collect any information about the influence ofSWNTs. On the other hand, the signal affected by the SWNTs, after adouble passage of light through them, determines the squaretransmittance (T²) of the SWNTs, so that a measurement of thetransmittance of SWNTs in the whole 200-2500 nm range can be achieved.

In FIG. 8A a typical spectrum of the transmittance T as a function ofthe wavelength is shown for a sample. This graph exhibit a set ofintense absorption peaks at 1719 nm (0.72 eV), 1912 nm (0.65 eV), 2131nm (0.58 eV) and 2305 nm (0.54 eV), two weaker peaks in the 1200-1400 nm(1.0-0.9 eV) range and a wide band below 1100 nm (1.12 eV) without anypeak.

A tight binding calculation, devised for zig-zag (n,0) nanotubes revealsthat the presence of these peaks is consistent with light absorptionoccurring at the gap of the semiconducting SWNTs, as shown in FIG. 8A.On the other hand if this absorption is determined by SWNTs, because ofthe continuous distribution of diameters of the nanotubes produced byarc discharge, we would expect a set of three continuous bands centeredat 0.6 eV, 1.2 eV and 1.6 eV (these bands are determined by the gap ofmetallic nanotubes), respectively.

But there is no band centered at 1.6 eV and we observe a discretedistribution of peaks that fall within the other two bands.

Without being limited to it, an explanation is being advanced herein toexplain the absence of the absorption peaks that was expected.

In fact, for example, though it is unlikely that, starting from the sootwe purchased, we have been able to select some diameters or chiralitiesduring the purification process or before, it is likely that thepurification process can introduce some defects in the SWNTs,determining a rugged structure.

How a rugged structure can deteriorate the absorption properties ofSWNTs is not known, nevertheless it is quite unlikely that this processeliminates entirely the contribution by the metal nanotubes and hencethe band at 1.6 eV.

On the other hand in order to have an absorption in semiconducting SWNTswe need to evidence their gaps. This can be obtained by eliminating thepresence of electrons in the conduction band of the semiconducting SWNTsand will be achieved if some holes are available to eliminate theelectrons. Nevertheless, the oxidation process cannot selectivelyintroduce holes in the SWNTs according to their chirality.

As a consequence, though holes' injection into the nanotubes is anessential process in order to give an explanation to all the sets ofexperimental data that we gained in the UV-vis and near IR region, adeeper consideration about what can introduce holes into the SWNTs mustbe done.

The answer to this question is given by the shoulder that may beobserved in FIG. 8A at 1100 nm which corresponds to an energy of 1.12 eVwhich is attributable to the energy gap of silicon.

The presence of this signature reveals the role solved by Si (p type).Silicon p, which we used as a substrate, is a source of holes close tothe SWNTs that can be injected into the tubes marking the presence ofthe gap. In particular, all those SWNTs nanotubes that have a gapgreater than that of Si do not give rise to an absorption peak, since inthis case their gap is covered by the gap of Si and the band at 1.6 eVwill be lacking.

On the other hand, if the energy gap of SWNTs falls within the energygap of Si we observe the absorption peaks. In this case holes' injectioninto the nanotubes is determined by the difference in the work functionsbetween a SWNT of a determined chirality and that of silicon as depictedin FIG. 1. In this picture the contact between the two semiconductors,Si and the semiconducting nanotubes, is sketched.

In particular, the work function of Si has been considered as 4.35 eVwith respect to the conduction band of SiO₂ (a layer of 1-2 nm of nativeoxide naturally forms on the Si surface), whereas the work function of aSWNT has been considered equal to 4.8 eV as an average.

Since the work functions of Si and SWNTs are comparable, what occurs isthat for some tubes this process can be favorable and for other onesunfavorable to the holes injection, thereby determining the presence orthe lack of absorption peaks in the near IR where we should have thepresence of two bands.

In conclusion, the optical properties of SWNTs can be investigated byusing Si p-type as a substrate. The presence of Si acts selectively byevidencing the absorption at the gap of those semiconducting SWNTs witha gap within that of Si. In particular, Si p can inject holes into thenanotubes according to the difference in work functions of Si and SWNTs.The process of holes injection suggests the possibility to build up newoptoelectronic devices.

The light emitting device and corresponding manufacturing method of thepresent invention solve the problem of exploiting the optoelectronicsproperties of semiconducting SWNTs.

In particular it has been evidenced experimentally the possibility thatholes can be injected from Si to semiconducting SWNTs. As a consequenceof this property it has been shown a method to build light emittingdiodes based on semiconducting SWNTs and consequently by implementing aFabry Perrot cavity a laser based on semiconducting SWNTs.

The exploitation of SWNTs in the field of optoelectronics will carrymany benefits to the field, because SWNTs have direct band gap and canbe grafted on silicon. Moreover it has been proved that SWNTs can havealso ultra fast optical switching properties, which will allowtransmitting optical signals at high speeds.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A method comprising: forming a first electrode; depositing singlewalled carbon nanotubes (SWNTs) on a surface of the first electrode;forming an insulating layer of a semiconductor material, the SWNTs beingembedded in the insulating layer; and forming a second electrode on theinsulating layer.
 2. The method of claim 1 wherein the first electrodeis p-doped silicon, and the second electrode is n-doped silicon
 3. Themethod of claim 2 wherein forming the insulating layer includes growinga silicon oxide layer on the first electrode.
 4. The method of claim 2wherein forming the insulating layer includes growing a silicon layer onthe first electrode.
 5. The method of claim 1 wherein forming theinsulating layer includes depositing a silicon oxide layer.
 6. Themethod of claim 1 wherein the insulating layer is no more than 10 nmthink.
 7. A method of forming a nanojunction comprising: forming a firstelectrode on a silicon substrate; depositing semiconductive singlewalled carbon nanotubes (SWNTs) on a surface of the first electrode;growing an insulating layer on the silicon substrate, the insulatinglayer covering the SWNTs; and forming a second electrode on theinsulating layer.
 8. The method of claim 7 wherein the insulating layeris a silicon oxide layer.
 9. The method of claim 7 wherein theinsulating layer is a silicon layer.
 10. The method of claim 7 whereinthe first electrode is p-doped silicon, and the second electrode isn-doped silicon.
 11. The method of claim 7 wherein the insulating layeris no more than 10 nm thick.
 12. A method of forming a Fabry-Perrotmicrocavity comprising: forming a first Bragg reflector havingalternating silicon and silicon oxide layers; forming a second Braggreflector having alternating Si and SiO₂ layers; and forming ananojunction positioned between the first Bragg reflector and the secondBragg reflector, wherein the nanojunction comprises an n-doped siliconelectrode, a p-doped silicon electrode, and a semiconductor insulatinglayer including single walled carbon nanotubes (SWNTs) positionedbetween the n-doped silicon electrode and the p-doped silicon electrode.13. The method of claim 12 wherein forming the first Bragg reflectorcomprises annealing a silicon oxide layer to the n-doped siliconelectrode of the nanojunction.
 14. The method of claim 12 whereinforming the second Bragg reflector comprises annealing a silicon oxidelayer to the p-doped silicon electrode of the nanojunction.
 15. Themethod of claim 12 wherein the semiconductor insulating layer is no morethan 10 nm thick.
 16. The method of claim 12 wherein the semiconductorinsulating layer is formed of silicon oxide.
 17. The method of claim 12wherein the semiconductor insulating layer is formed of silicon.